[commit: ghc] master: compiler: Do not look up fail in RnExpr if bind pattern is irrefutible. (2fcb5c5)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/2fcb5c5c3f6c5a5936eeb5dc07b476e5737f12ad/ghc

>---------------------------------------------------------------

commit 2fcb5c5c3f6c5a5936eeb5dc07b476e5737f12ad
Author: Aaron Friel <mayreply at aaronfriel.com>
Date:   Thu May 11 15:41:22 2017 -0400

    compiler: Do not look up fail in RnExpr if bind pattern is irrefutible.
    
    Adds a check in `rnStmt`, in sub-expr `getFailFunction`, to determine if
    the pattern of a bind statement is irrefutible. If so, skip looking up
    the `fail` name.
    
    Reviewers: austin, bgamari
    
    Reviewed By: bgamari
    
    Subscribers: rwbarton, thomie
    
    GHC Trac Issues: #13649
    
    Differential Revision: https://phabricator.haskell.org/D3553


>---------------------------------------------------------------

2fcb5c5c3f6c5a5936eeb5dc07b476e5737f12ad
 compiler/rename/RnExpr.hs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/compiler/rename/RnExpr.hs b/compiler/rename/RnExpr.hs
index 154e270..ce22784 100644
--- a/compiler/rename/RnExpr.hs
+++ b/compiler/rename/RnExpr.hs
@@ -833,6 +833,10 @@ rnStmt ctxt rnBody (L loc (BindStmt pat body _ _ _)) thing_inside
 
         ; xMonadFailEnabled <- fmap (xopt LangExt.MonadFailDesugaring) getDynFlags
         ; let getFailFunction
+                -- If the pattern is irrefutible (e.g.: wildcard, tuple,
+                -- ~pat, etc.) we should not need to fail.
+                | isIrrefutableHsPat pat
+                                    = return (noSyntaxExpr, emptyFVs)
                 -- For non-monadic contexts (e.g. guard patterns, list
                 -- comprehensions, etc.) we should not need to fail.
                 -- See Note [Failing pattern matches in Stmts]